1. Field of the Invention
The present invention relates to a compatible optical pickup which can record and/or reproduce information on/from a plurality of optical discs having different formats by using light beams having a plurality of wavelengths, and more particularly, to a compatible optical pickup to correct chromatism generated in an objective lens due to a change in an output of the light source when a record and/or reproduction mode is switched with respect to a high density optical disc.
2. Description of the Related Art
In an optical recording and/or reproducing apparatus to record or reproduce information with respect to an optical disc by using a light spot formed by an objective lens, a capacity to record is determined by a size S of the light spot. The size S of the light spot is proportional to a wavelength λ of a light beam and inversely proportional to numerical aperture (NA) of the objective lens, as shown by the following Equation 1:S∝λ/NA  Equation 1
Accordingly, to reduce the size S of a light spot focused on an optical disc to achieve high densification of the optical disc, it can be seen from Equation 1 that a short wavelength light source such as a light source emitting a blue-violet laser beam and an objective lens having an NA of 0.6 or more are necessary.
Thus, an optical pickup for a next generation DVD, so called an HD-DVD (high definition-digital versatile disc), that is currently under development, adopts a light source to emit the blue-violet laser beam and the objective lens having the NA of 0.6 or more, to obtain a higher information recording density than the information recording density of a conventional optical disc in a CD or a DVD family, by reducing the size S of the light spot formed on the optical disc.
However, an optical material such as glass and plastic used for the objective lens of a typical optical pickup exhibits a very sharp change in a refractive index in a range of a wavelength shorter than 650 nm.
Table 1 shows a change in the refractive index according to wavelength of M-BaCD5N manufactured by HOYA, which is used as a material for the glass to mold the objective lens.
TABLE 1Change in refractive index of HOYA'sChange in wavelengthM-BaCD5N glass650 nm → 651 nm0.000038405 nm → 406 nm0.000154
As can be seen from Table 1, the optical material shows a great change in the refractive index by about four times in a range of a blue-violet wavelength compared to a red wavelength used for the optical pickup for the DVD, with respect to a small change in wavelength by about 1 nm.
A sharp change in the refractive index of the optical material in a short wavelength mainly causes deterioration of performance, according to a defocus in a recordable high density optical pickup capable of repeatedly switching between recording and reproduction. That is, the optical pickup uses a recording optical power and a reproduction optical power which are different from each other. In general, if the output of a light source is increased, the wavelength of the light beam emitted from the light source becomes elongated. Chromatism generated in the objective lens due to a change in the wavelength according to a change in the optical power during a mode switch between recording and reproduction causes the defocus (i.e., defocus according to a mode hopping). Here, the mode hopping refers to a phenomenon in which the wavelength increases or decreases intermittently depending on conditions of the light source, for example, a change in temperature. When an input current is increased to increase the optical power, the temperature in the light source increases due to internal heat. Thus, the mode hopping occurs.
Although the defocus due to the change in the wavelength can be corrected by adjusting the objective lens by an actuator, because it takes a relatively long time to follow the change in the wavelength by driving the actuator, the quality of the reproduced and recorded signals is deteriorated during the above time. The defocus generated when the optical power is increased to record causes a lack of recording optical power while the defocus increases jitter when the optical power is decreased for reproduction.
That is, when the output of the light source is increased to record information on the optical disc, the wavelength of the light beam emitted from the light source is elongated to, for example, 406 nm, so that the light spot is defocused and recording cannot be performed appropriately until the actuator follows the defocus. When the output of the light source is decreased for reproduction, the wavelength of the light source is shortened to, for example, 405 nm. In this case, because the actuator is in a state of following the elongated wavelength, defocus is generated again. When the defocus is generated, jitter increases in a reproduction signal.
Thus, the optical pickup for a high density capable of recording in which recording and reproduction are repeated needs to have an optical system structure capable of restricting generation of chromatism or compensating therefore, even when the wavelength of the light beam emitted from the light source changes when recording and reproducing.
In the meantime, assuming that an inclination angle of the optical disc is q, the refractive index of the optical disc is n, a thickness of the optical disc is d, and the NA of the objective lens is NA, a coma aberration W31 generated by the inclination of the optical disc can be expressed by the following Equation 2:
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              Equation        ⁢                                  ⁢        2            
Here, the refractive index and the thickness of the optical disc correspond to those of the optical medium from each of light incident surfaces of the recording surface.
Considering Equation 2, to secure an allowance for the inclination of the optical disc, as NA the of the objective lens increases for the high density disc, the thickness of the optical disc needs to be reduced. Accordingly, the thickness of the DVD is reduced to 0.6 mm from 1.2 mm of the CD. The thickness of the HD-DVD may be reduced to 0.1 mm. Of course, the NA of the objective lens of the DVD is increased to 0.6 from 0.45 of the CD. In the case of the HD-DVD, the NA of the objective lens may increase to 0.85. Also, considering the recording capacity of the HD-DVD, the blue-violet light source is very likely to be adopted. In developing the optical disc having such a new specification, compatibility with a conventional optical disc is important.
For example, because a reflectance of a DVD-R or a CD-R of conventional optical discs which is recordable only one time is drastically lowered according to the wavelength, use of the light source of a wavelength of 650 nm and 780 nm is necessary. Thus, considering the compatibility with the DVD-R and/or the CD-R, a compatible optical pickup to record and/or reproduce the HD-DVD needs to adopt two or three light sources having different wavelengths.
Here, the compatible optical pickup adopting a plurality of light sources having different wavelengths may include a single objective lens considering various merits such as a size of an apparatus, easiness in assembly, and cost.
However, design and manufacturing of the objective lens having a high NA of 0.85 into one unit requires high technology and it is difficult to have a working distance lengthy like the objective lens for the DVD, while maintaining the high NA. In the field of the present technology, the objective lens is typically designed to have the working distance of 0.6 mm with respect to the blue-violet light source and the HD-DVD having a thickness of 0.1 mm. When the light beam emitted from the light source for the DVD and the light source for the CD is focused by the objective lens having the working distance of 0.6 mm with respect to the HD-DVD and forms the light spot on the DVD and the CD, the working distance is 0.32 mm and −0.03 mm, respectively, which means that the CD collides with the objective lens.
When the light beam emitted from the light source for the CD is input to the objective lens in a form of a divergent light beam by reducing a distance between the light source for the CD and a collimating lens, the working distance can be secured. However, in this case, because the optical system is a finite optical system, an aberration characteristic, according to the movement of the actuator in a radial direction, is sharply deteriorated which is not desirable.
In summary, a recordable high density optical pickup to repeatedly record and reproduce by using the light source having a shorter wavelength than the red wavelength needs an optical system structure which can restrict or compensate for the defocus during the switching between recording and reproduction modes. Also, the high density optical pickup needs to have lengthy working distance with respect to the light beam for the DVD and/or the CD considering the compatibility with a relatively low density DVD and/or CD.